A method of the type described above is known from the Japanese patent application Sho-59-94526. In this, 95.0 to 99.5% by weight chromium oxide powder is mixed with 5.0 to 0.5% by weight titanium oxide powder. Intermediate units are formed from this mixture and fired in a first firing operation. The firing is carried out in a reducing atmosphere in order to achieve a solid solution of the titanium oxide in the chromium oxide. The fired intermediate units are subsequently crushed by mechanical means, i.e. ground actually. The ground material is measured into sieve fractions which are are then recombined according to the batch composition. The units which are to be subjected to firing in a second firing operation are then formed from this material. The second firing operation is carried out in an oxidizing atmosphere. Good mixing of the chromium oxide powder with the titanium oxide powder can be achieved through spray drying. This known two-stage production technique has the advantage that refractory bricks are available after the second firing operation and that said bricks exhibit relatively little firing shrinkage. However, this firing shrinkage is still more than 1% and is in fact in the order of magnitude of 1.2 to 2.8%. Another advantage is that a good microstructure results in which the titanium oxide is dissolved in the chromium oxide. A disadvantage is the particularly expensive production involved in the two-stage firing procedure, whereby all 100% of the raw materials needs to be fed through the first and also through the second firing operation. The spalling resistance of the refractory bricks produced is only good in certain circumstances. Cracks in the bricks do appear at the first chill test. If, however, bricks were to be manufactured from porous material, then the spalling resistance would be better. In the second firing operation, which takes place in an oxidizing atmosphere, there results not only the bridging over the titanium oxide with the advantageous microstructures described, but also a certain proportion of hexavalent chromium oxide which is harmful to health. This proportion is particularly high when firing in the temperature range between 1200.degree. and 1600.degree. C. which, however, cannot be avoided because of the microstructure.
A production method for dense chromium oxide bricks is known from the publication "Properties of the Chromic Oxide Refractories", Kainarskii & Degtyarva, Ogneupory, No. 3, pp. 36-41, March 1977 in which the moulded raw materials are fired in a single-stage firing operation in a reducing atmosphere. During this sintering takes place. This method is particularly cost-effective because the firing costs are only incurred once. Bricks with a higher apparent density can be produced which also exhibit a low open porosity. Such bricks possess particulary high corrosion resistance with regard to molton glass. A disadvantage with this method is the high firing shrinkage which lies in the order of magnitude of 10 to 15%. The spalling resistance is also comparatively poor. As a result of its density, the material exhibits a relatively high thermal conductivity so that its use in the construction of kilns demands special insulation measures.
Furthermore, a production method for porous bricks with low density, improved spalling resistance and low thermal conductivity is known from "Low Density and Low Thermal Conductivity Chrome Oxide Refractories for Glass-Melting Furnaces for Fiber Production", Krivoruchko et al, Ogneupory, No. 4, pp. 45-49, April 1981 in which the material is fired in a single-stage firing operation in an oxidizing atmosphere in conjunction with an opening material. This opening material leaves behind cavities, thus increasing the porosity so that, advantageously, bricks with low density and high porosity can be manufactured. Owing to the low specific weight, these bricks are also advantageous in terms of material consumption. However, they cannot be used in direct contact with the molton glass because the porous bricks do not provide enough resistance to the glass.